Technical Field
The present disclosure relates to a differential amplifier with common mode compensation circuit.
Description of the Related Art
General-purpose operational amplifiers are compensated in frequency in order to be stable in unit-gain configuration, i.e., in the worst-case condition of closed-loop gain.
Instead, an amplifier that forms part of an integrated circuit is typically designed to have a closed-loop gain correlated to a specific need, with a gain predefined as a function of the application chosen. In the case of small input signals, the amplifier typically presents a high closed-loop gain, for example, between 10 and 100. Known amplifiers with gain higher than one may have an inverting configuration or a non-inverting configuration, with appropriate choice of the value of resistance on the feedback branch.
In order to handle the limited supply voltages available in portable devices, and to reject common mode or supply noise or interference, typically fully differential or fully balanced amplifiers are used. A fully differential amplifier, with high gain, of an inverting type, is illustrated in FIG. 1. An amplifier 1 of this type includes four input terminals 1a-1d and two output terminals 1e, 1f. The terminal 1a is a non-inverting terminal, and the terminal 1b is an inverting terminal. An input signal (voltage) Vin, is applied between the input terminals 1a and 1b, by interposition of resistors, which have the same value of resistance R1. The output terminals 1c and 1d are coupled in feedback mode to the input terminals 1a and 1b, respectively, by respective resistors, which have values of resistance R2 that are the same as one another. The difference between the output terminals Vout represents the useful differential signal, i.e., the input signal Vin amplified by the ratio R2/R1.
A common mode feedback circuit is implemented, in a per se known manner, by integration of a common mode amplifier 4, configured to fix the value of the half-sum of the outputs 1e and 1f, which is defined as “common mode”, at a pre-set value VCM. The input 1c of the amplifier 1 is coupled between an input of the common mode amplifier 4 and the output terminals 1c, 1d by the resistors 6, 8. The input 1d of the amplifier 1 is biased at a fixed voltage VCM, chosen as desired for fixing the common mode of the differential output signal of the amplifier (VCM is chosen, for example, in a range comprised between a supply voltage VCC and a voltage of a reference node, e.g., ground reference equal to 0 V—for example, VCM is equal to VCC/2).
Frequency compensation of the differential path is carried out at a frequency that is β times lower than the unit-gain frequency of the amplifier, where β is the feedback factor given by R1/(R1+R2). This technical solution has the function of preventing the design of the amplifier from being over-sized in terms of electric-power consumption and area of silicon. However, the common mode feedback path is compensated in frequency at the unit-gain frequency, given that its feedback factor is 1. With reference to FIG. 2, the curve GLOOP_CMFB represents the loop gain for the common mode amplifier 4 (GLOOP_CMFB=ACMBF), whereas the curve GLOOP_DIFF represents the loop gain for the differential amplifier 1 (GLOOP_DIFF=βADIFF). As may be noted from the Bode diagram of FIG. 2, the gain of the common mode amplifier 4 and of the differential amplifier 1 follow a similar pattern, with respective poles at frequency values f1′ ˜f1″, and f2′ ˜ f2″, but different gain values G1>G2 on the input. It follows that, whereas the curve of GLOOP_DIFF intercepts the frequency axis at f3<f2″ with a slope of 20 dB/dec, the curve of GLOOP_CMFB intercepts the frequency axis at f4>f2′ (i.e., after the second pole) and with a slope of 40 dB/dec. This causes an instability of the common mode amplifier 4.